Disposable urea sensor and system for determining creatinine and urea nitrogen-to-creatinine ratio in a single device

ABSTRACT

A disposable urea sensor has a laminated body having a fluid sample inlet end and an electrical contact end, a fluid sample inlet, a substantially flat sample chamber in communication between the fluid sample inlet and a vent opening, the sample chamber being adapted to collect a fluid sample through the fluid sample inlet, a working electrode and a reference electrode within the sample chamber, and a reagent matrix disposed on the working electrode wherein the reagent matrix contains urease.

This application is a Divisional application of Ser. No. 11/164,935,filed on Dec. 12, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrochemical sensors thatcan be used for the quantification of a specific component or analyte ina liquid sample. Particularly, the present invention relates to adisposable electrochemical sensor for measuring urea (or BUN, Blood UreaNitrogen) in biological fluids such as blood. More particularly, thepresent invention relates to a system for simultaneously determiningcreatinine and BUN-to-creatinine ratio in a single device. Still moreparticularly, the present invention relates to a device that can beemployed to perform assays of urea, creatinine and blood ureanitrogen-to-creatinine ratio in a small volume liquid sample (around 1μL).

2. Description of the Prior Art

In the liver, urea is synthesized from ammonia produced as a result ofde-amination of amino acids. This biosynthetic pathway is the main meansof disposal of excess nitrogen by the body. Therefore, the measurementof blood urea nitrogen (BUN) is one of the most commonly used screeningtests for the evaluation of kidney function. High urea concentration inblood is toxic to the body. Malfunctioning kidneys will have difficultyin getting rid of the excess urea.

There are currently several hundred thousand people with endstage renaldisease in the United States. These people require regular hemodialysis.Urea is generally accepted to be the best marker for evaluating thelevel of uremic toxins. Hemodialysis is a procedure that has been usedas a means for reduction of urea in blood. Currently, most dialysisclinics use the simple index of time of dialysis to determine theadequacy of dialysis. Drawing blood to accurately measure the level ofurea in the blood is done infrequently. During this procedure, blood isdrawn and sent to a central laboratory for the measurement of urea.Turnaround times for these samples can be quite long.

It has long been desired to have a sensor which could monitor theprogress of the dialysis procedure in “real-time”, thereby assuring thatthe procedure was complete and obviating the need for using clinicallaboratories. A further enhancement of this general principle is a homemonitor, which would allow at-home testing to monitor the peritonealdialysis and home hemodialysis. Such a home monitor is similar, inprinciple, to devices used for blood glucose testing by diabetics. Thisdevice would require a blood droplet sample by using a finger prick.

There has been substantial efforts in the prior art to provide a sensorwhich would satisfy the above-mentioned needs, and variouselectrochemical sensors for detecting urea in body fluids have beenproposed. These prior art devices included the use of potentiometricdevices, specifically, the common pH, ammonium, or ammonia gas sensingelectrodes. They also included a biosensor for urea by depositing acoating containing the enzyme urease immobilized over the interdigitatedconductive members.

Other publications teach of the need for dialysis or blood ureamonitoring, and various mechanical systems which could be used in such asystem, albeit with different or unspecified urea detection (sensor)methods than those of the present invention. The prior approaches alsodescribe various methods of enzyme (urease) immobilization to varioussubstrates for detection of urea in blood or in dialysate fluid. Forexample, Cozzette et. al. (U.S. Pat. No. 5,466,575; U.S. Pat. No.5,063,081) described a microfabricated BUN sensor, which is based on thepotentiometric measurement of ammonium ions resulting from an enzymaticreaction. None of the previous approaches, however, describes acombination of disposable, small volume, mass fabricated sensors withdissolvable reagent matrix containing the enzyme urease.

The prior known sensors have not enabled the desired point-of-care bloodtest for blood urea, thereby assuring that the procedure was complete.In the absence of a reliable point-of-care blood test for blood urea, todetermine whether the dialysis process is long enough to rid the excessurea, it will be desirable to have a sensor which can be used to monitorthe progress of the treatment simply by measuring the blood ureaconcentration from a drop of blood obtained from a finger prick.

It is also well known that creatinine is a waste product derived fromcreatine and excreted by the kidneys. The analytical determination ofcreatinine in biological samples is a widely used and extremelyimportant test for renal dysfunction. Measurements of creatinine inwhole blood, serum or urine may also be used as indices in the diagnosisand treatment of other disorders such as muscular dystrophy andhypothyroidism. Thus, the creatinine assay has been widely recognized ashaving vital medical significance. Further, dietary changes have little,if any, influence on the creatinine concentration in blood and urine.

The BUN-to-creatinine ratio is an important index used by healthprofessionals to predict what conditions may be causing abnormal BUN andcreatinine levels and decreased kidney function. The normal values forthe BUN-to-creatinine ratio are typically in the 10:1 to 20:1 range forpersons over 12 months of age and up to 30:1 for infants less than 12months of age. A high BUN value can indicate kidney injury or diseases,such as, diabetes, high blood pressure, kidney stone, or tumor. It canalso be caused by reduced blood flow to the kidneys caused bydehydration or heart failure. Certain medications may also cause highBUN values.

High BUN-to-creatinine ratios occur with sudden kidney failure, whichmay be caused by conditions such as shock or severe dehydration. Anobstruction in the urinary tract can also cause an elevatedBUN-to-creatinine ratio. A very high BUN-to-creatinine ratio may becaused by bleeding in the digestive tract or respiratory tract.

On the other hand, a low BUN value may be caused by a diet very low inprotein, malnutrition, or severe liver damage. Drinking excessiveamounts of liquid may cause overhydration and lead to a low BUN value.

Low BUN-to-creatinine ratios may be associated with a diet low inprotein, severe muscle injury called rhabdomyolysis, pregnancy,cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion(SIADH). SIADH sometimes occurs with lung disease, cancer, diseases ofthe central nervous system, and the use of certain medications.

Therefore, what is needed is a disposable sensor capable of measuringurea in a small volume of body fluid sample such as blood. What is alsoneeded is a disposable sensor capable of measuring urea in a smallvolume of body fluid such as blood obtained by lancing the skin of auser. What is further needed is a system for determining the blood urea,creatinine and blood urea nitrogen-to-creatinine ratio in a small volumeof body fluid using a single device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a disposable ureasensor capable of measuring the urea concentration in a small volume ofbody fluid. It is another object of the present invention to provide adisposable urea sensor that is capable of measuring the ureaconcentration in a small volume of body fluid such as blood obtained bylancing the skin of the user. It is a further object of the presentinvention to provide a disposable urea sensor that has a fast responsetime. It is yet another object of the present invention to provide adisposable sensor for use in determining urea, creatinine andBUN-to-creatinine ratio in a body fluid sample. It is still anotherobject of the present invention to provide a system for determiningurea, creatinine and BUN-to-creatinine ratio in a blood sample with asingle disposable device.

The present invention provides these and other objectives in thefollowing described embodiments.

In the first embodiment of the present invention, the sensor of thepresent invention uses a 4-layer laminated construction, similar to theglucose sensor, which has been disclosed in the following (U.S. Pat.Nos. 6,767,441; 6,287,451; 6,837,976), which are incorporated herein byreference.

In one aspect of the first embodiment, the sensor of the presentinvention has a laminated, elongated body having a sample fluid channelconnected between an opening on one end of the laminated body and a venthole spaced from the opening. Within the fluid channel lie at least oneworking electrode and a reference electrode (or a counter electrode).The arrangement of the at least one working electrode and the referenceelectrode (or the counter electrode) is not important for purposes ofthe results obtained from the sensor. The at least one working electrodeand the reference electrode are each in electrical contact with separateconductive paths. The separate conductive paths terminate and areexposed for making an electrical connection to a reading device on theend opposite the open channel end of the laminated body.

In another aspect of the first embodiment, the laminated body has a baseinsulating layer made from a plastic material. At least two conductivepaths are delineated on the base insulating layer. The conductive pathsmay be deposited on the insulating layer by screen printing, by vapordeposition, or by any method that provides for a conductive layer thatadheres to the base insulating layer. The conductive paths can beindividually disposed on the insulating layer, or a conductive layer maybe disposed on the insulating layer followed by etching/scribing therequired number of conductive paths. The etching process may beaccomplished by chemically, mechanically scribing lines in theconductive layer, using a laser to scribe the conductive layer intoseparate conductive paths, or by any means that will cause a breakbetween and among the separate conductive paths required by the presentinvention. Conductive coatings or layers that may be used are coatingsof nickel, gold, tin oxide/gold, palladium, other noble metals or theiroxides, or carbon film compositions. The preferred conductive coatingsare gold film or a tin oxide/gold film composition.

In a further aspect of the first embodiment of the present invention,the laminated body has a first middle insulating layer, also called areagent holding/electrode area defining layer, on top of the baseinsulating layer and the conductive paths. The first middle layer, orreagent holding layer, contains at least two opening for one or moreworking electrodes and one or more reference electrodes. Each openingcorresponds to and exposes a small portion of an electrode surface. Theopenings for the one or more working electrodes may be the same ordifferent size, but preferably the same size. The opening for the one ormore reference electrodes may be the same or different size as theopenings for the one or more working electrodes, but preferably the samesize. The placement of all of the openings is such that they will be allpositioned within the sample fluid channel described above. The firstmiddle insulating layer is also made of an insulating dielectricmaterial, preferably plastic, and may be made by die cutting thematerial mechanically, or with a laser, and then fastening the materialto the base layer. An adhesive, such as a pressure-sensitive adhesive,may be used to secure the first middle insulating layer to the baselayer. Adhesion may also be accomplished by ultrasonically bonding thefirst middle layer to the base layer. The first middle insulating layermay also be made by screen printing an insulating material or by bindinga photopolymer over the base layer.

In yet another aspect of the first embodiment, the laminated body alsohas a second middle insulating layer, called a channel-forming layer, ontop of the first middle layer. The second middle layer, orchannel-forming layer, is also made of a plastic insulating material andcreates the sample fluid channel of the laminated body. It contains aU-shaped cutout on one end which overlays the openings on the firstmiddle layer with the open end corresponding to the open end of thelaminated body described earlier. A double coated, pressure-sensitiveadhesive tape may be used as the second middle layer.

In yet another aspect of the first embodiment, the laminated body of thepresent invention has a top layer with a vent opening and preferably anentrance notch. The vent opening is located such that at least a portionof the vent opening overlays the bottom of the U-shaped cutout of thesecond middle insulating layer. The vent allows air within the samplefluid channel to escape as the sample fluid enters the sample entranceend of the laminated body. The notch is located at the sample entranceend. The sample fluid generally fills the sample fluid channel bycapillary action. In small volume situations, the extent of capillaryaction is dependent on the hydrophobic/hydrophilic nature of thesurfaces in contact with the fluid undergoing capillary action.Capillary forces are enhanced by either using a hydrophilic insulatingmaterial to form the top layer, or by coating at least a portion of oneside of a hydrophobic insulating material with a hydrophilic substancein the area of the top layer that faces the sample fluid channel betweenthe sample entrance end of the laminated body and the vent opening ofthe top layer. It should be understood that an entire side of the toplayer may be coated with the hydrophilic substance and then bonded tothe second middle layer.

In yet another aspect of the first embodiment, one opening containselectrode material for the working electrode (W) loaded with ureasensitive enzyme (urease) and other ingredients, and one opening for thereference electrode (R). The positional arrangement of the workingelectrode and the reference electrode in the channel is not critical forobtaining usable results from the electrochemical sensor. The possibleelectrode arrangements within the sample fluid channel may be W-R orR-W, with the arrangement listed as the arrangement of electrodes wouldappear from the sample entrance end of the laminated body to the ventopening. The preferred position was found to be W-R; that is, as thesample fluid entered the entrance open end of the laminated body, thefluid would cover W first, then R. The preferred position obviatesreliability and accuracy problems due to an insufficient sample fluidsize. The working electrode and the reference electrode are each inelectric contact with separate conductive paths, respectively. Theseparate conductive paths terminate and are exposed for making anelectric connection to a reading device on the end opposite the sampleentrance end of the laminated body.

In a further aspect of the first embodiment, the working electrode isloaded with a mixture of at least an enzyme (urease), and optionallywith one or more of a surfactant, a polymer binder, and a buffer. Thereference electrode may be loaded with the same mixture as the workingelectrode, with an addition of a redox mediator (oxidized form), or amixture of both reduced and oxidized form of redox mediators.Preferably, the reference electrode opening is loaded with an oxidizedform of redox mediator, such as potassium ferricyanide, along with otheringredients. The reference electrode opening could also be loaded withan Ag/AgCl layer (e.g., by applying Ag/AgCl ink or by sputter-coatingsilver or an Ag/AgCl layer) or other reference electrode materials.

In a yet further aspect of the first embodiment, when a liquid sample isapplied to the sensor of the present invention, the sample fills up theentire channel and covers both working electrode and reference electrode(or counter electrode). When a positive potential (e.g. 0.4 V vs. thereference electrode; if the reference electrode opening is loaded with aredox mediator in oxidized form, such as potassium ferricyanide) isimposed across the working and reference electrodes, surprisingly, anoxidation current signal, related to the urea concentration in thesample, is observed. The current response is proportional to the ureaconcentration over the entire concentration range of clinicalimportance. The detail mechanism for the involved enzymatic andelectrochemical reactions and the urea signal are not clear. It has beenfound that the urease loading for the working electrode is essential forthe generation of the electrochemical signal of urea in the sample. Inmost enzyme-based electrochemical sensors, e.g. glucose strips, a redoxmediator or oxygen is always involved at the working electrode in orderto generate the useful electrochemical signal. In contrast to theconventional enzyme-based electrochemical sensors, the redox mediatorsat the working electrode of the BUN sensor of the present invention arenot required to obtain the electrochemical response of urea although theredox mediators, such as potassium ferricyanide, can be included at theworking electrode. If the working electrode contains a mediator, anoxidized form of the redox mediators is preferred. For example, both theworking electrode and the reference electrode are loaded with the samemixture of at least an enzyme (urease), at least a redox mediator, andoptionally with one or more of a surfactant, a polymer binder, and abuffer. The mixture covers either the working electrode and referenceelectrode openings to form two separate chemistry spots, or the wholechannel including the two electrode openings to form a layer of drychemistry. In this case, an oxidized form of the redox mediators ispreferred.

In the second embodiment of the present invention, the sensor of thepresent invention has a similar structure to the first embodiment, butit has one working electrode, one reference electrode and one blankelectrode. The working electrode is loaded with a urea sensitive enzyme(urease) and other ingredients; the blank electrode is loaded with asimilar chemistry except without adding a urea sensitive enzyme. Such athree-electrode system not only possesses the feature of the firstembodiment, but also the capability of eliminating interference from anyoxidizable species in the sample, such as ascorbic acid, acetaminophen,uric acid, etc.

In one aspect of the second embodiment, at least three conductive pathsare delineated on the base insulating layer. The first middle layer, orreagent holding layer, contains at least three openings for one workingelectrode, a reference electrode and one blank electrode.

In another aspect of the second embodiment, one opening containselectrode material for the working electrode (W) loaded with urease andother ingredients; one for the blank electrode (B) loaded with similarchemistry to W, without adding urease; and one for the referenceelectrode (R). The positional arrangement of the working electrode, thereference electrode and the blank electrode in the channel is notcritical for obtaining usable results from the electrochemical sensor.The preferred position was found to be W1-R-B; that is, as the samplefluid entered the entrance open end of the laminated body, the fluidwould cover W first, then R, then B.

In the third embodiment of the present invention, the sensor of thepresent invention has a similar structure to the first embodiment, butwithout the first middle layer; i.e., the other three layers are thesame as in the first embodiment. The detail of such a 3-layer layout hasbeen disclosed in U.S. Pat. Nos. 6,258,229; 6,942,770, which areincorporated herein by reference. The U-shaped channel cutout is locatedat the sensor end (sample entrance end). The length, thickness and widthof the U-shaped channel cutout define the capillary channel size orvolume. The length and width of the U-shaped channel cutout, along withthe base conductive layer, define the areas of the working and referenceelectrodes.

In one aspect of the third embodiment, the working electrode (W) isloaded with at least a urea sensitive enzyme (urease), and optionallywith one or more of a polymer binder, one or more of a surfactant andone or more of a buffer. The reference electrode (R) is preferablycovered by the same reagent mixture as the working electrode with anaddition of at least a redox mediator. Instead of the reagent mixture,the reference electrode could also be covered with an Ag/AgCl layer(e.g., by applying Ag/AgCl ink or by sputter-coating silver or anAg/AgCl layer) or other reference electrode materials.

In the fourth embodiment of the present invention, the sensor of thepresent invention has a similar structure to the first embodiment, butit has two additional working electrodes: one loaded withcreatinine/creatine sensitive enzymes and other ingredients; the otherloaded with creatine sensitive enzymes and other ingredients. Such afour-electrode system, arranged in one channel, not only possesses thefeature of the first embodiment, but also the capability of measuringcreatinine and creatine in the sample. Thus, the ureanitrogen-to-creatinine ratio can be calculated. Details of thecreatinine/creatine sensor have been disclosed in U.S. Pat. No.6,767,441.

In one aspect of the fourth embodiment, at least four conductive pathsare delineated on the base insulating layer. The first middle layer, orreagent holding layer, contains at least four openings for three workingelectrodes and a reference electrode.

In another aspect of the fourth embodiment, one opening containselectrode material for the first working electrode (W1) loaded withurease and other ingredients, one for the second working electrode (W2)loaded with creatinine/creatine sensitive enzymes and other ingredients,one for the third working electrode (W3) loaded with creatine sensitiveenzymes and other ingredients, and one for the reference electrode (R).The positional arrangement of the working electrodes and the referenceelectrode in the channel is not critical for obtaining usable resultsfrom the electrochemical sensor. The preferred position was found to beW1-W2-R-W3; that is, as the sample fluid entered the open end of thelaminated body, the fluid would cover W1 first, then W2, then R, thenW3.

In still another aspect of the fourth embodiment, in order for thereference electrode to function properly for all of the three workingelectrodes, the reference electrode opening may be loaded with at leasta polymer, a surfactant, a buffer, with an addition of at least a redoxmediator (either reduced form, e.g. potassium ferrocyanide, or oxidizedform, e.g. potassium ferricyanide) or a mixture of a reduced form redoxmediator and an oxidized form redox mediator. Preferably, a mixture of areduced form redox mediator and an oxidized form redox mediator isloaded onto the reference electrode opening. Instead of the referencereagent mixture disclosed above, the reference electrode opening couldalso be loaded with an Ag/AgCl layer (e.g., by applying Ag/AgCl ink orby sputter-coating a silver or Ag/AgCl layer) or other referenceelectrode materials.

In the fifth embodiment of the present invention, the sensor of thepresent invention has a similar structure to the second embodiment, butit has two additional working electrodes, i.e., in the one singlechannel, there are at least four working electrodes and one referenceelectrode.

In one aspect of the fifth embodiment, at least five conductive pathsare delineated on the base insulating layer. The first middle layer, orreagent holding layer, contains at least five openings for the fiveworking electrodes and one reference electrode.

In another aspect of the fifth embodiment, the first working electrode(W1) is loaded with a urea sensitive enzyme (urease) and otheringredients; the second working electrode (W2) is loaded withcreatinine/creatine sensitive enzymes and other ingredients; the thirdworking electrode (W3) is loaded with creatine sensitive enzymes andother ingredients; the blank electrode (B) is preferably loaded with thesame chemistry as W1 without adding any enzyme. Such a five-electrodesystem (including the reference electrode), in a single device, not onlypossesses the feature of the second embodiment, having the capability ofeliminating interference from any oxidizable species in the sample, butalso possesses the capability of measurement of creatinine and bloodurea nitrogen-to-creatinine ratio.

In still another aspect of the fifth embodiment, in order for thereference electrode to function properly for all of the four workingelectrodes, the reference electrode opening may be loaded with at leasta polymer, a surfactant, a buffer, with an addition of at least a redoxmediator (either reduced form, e.g. potassium ferrocyanide, or oxidizedform, e.g. potassium ferricyanide) or a mixture of a reduced form redoxmediator and an oxidized form redox mediator. Preferably, a mixture of areduced form redox mediator and an oxidized form redox mediator isloaded onto the reference electrode opening. Instead of the referencereagent mixture disclosed above, the reference electrode opening couldalso be loaded with an Ag/AgCl layer (e.g., by applying Ag/AgCl ink orby sputter-coating a silver or Ag/AgCl layer) or other referenceelectrode materials.

In still another aspect of the fifth embodiment, the positionalarrangement of the working electrodes, the blank electrode and thereference electrode in the channel is not critical for obtaining usableresults from the electrochemical sensor. There are numerous combinationsfor the positional arrangement. For example, one possible arrangement isW1-B-R-W2-W3; that is, as the sample fluid enters the entrance open endof the laminated body, the fluid would cover W1 first, then B, then R,then W2, then W3.

In the sixth embodiment of the present invention, the sensor of thepresent invention has a similar structure to the fifth embodiment, butat least five conductive paths are delineated on the base insulatinglayer: one for the BUN working electrode (W1); one for thecreatinine/creatine working electrodes (W2); one for creatine workingelectrode (W3); one for the BUN reference electrode (R1) and one for thecreatinine/creatine reference electrode (R2). Accordingly, the firstmiddle layer, or reagent holding layer, contains at least five openingsfor three working electrodes and two reference electrodes.

In one aspect of the sixth embodiment, one opening contains electrodematerial for the first working electrode (W1) loaded with urease andother ingredients; one for the second working electrode (W2) loaded withcreatinine/creatine sensitive enzymes and other ingredients; one for thethird working electrode (W3) loaded with creatine sensitive enzymes andother ingredients; one for the BUN reference electrode (R1) and one forthe combined creatinine/creatine and creatine reference electrode (R2).The positional arrangement of the working electrodes and the referenceelectrodes in the channel is not critical for obtaining usable resultsfrom the electrochemical sensor. There are numerous combinations for thepositional arrangement. For example, one possible arrangement isW3-W2-R2-R1-W1; that is, as the sample fluid enters the entrance openend of the laminated body, the fluid would cover W3 first, then W2, thenR2, then R1, then W1.

In another aspect of the sixth embodiment, the BUN sensor (W1) andcreatinine/creatine sensors (W2/W3) have separate reference electrodes.The reference electrode for W1 will be similar to those described in thefirst embodiment; the reference electrode for W2/W3 is preferably loadedwith a reduced form of redox mediator, such as potassium ferrocyanide,and other ingredients as disclosed in U.S. Pat. No. 6,767,441.

In another aspect of the sixth embodiment, the BUN andcreatinine/creatine reference electrodes are loaded with at least aredox mediator, a polymer binder, a surfactant, and a bulking reagent.The redox mediators for the BUN reference electrode are preferably intheir oxidized form, such as for example potassium ferricyanide; whilethe redox mediators for the creatinine/creatine reference electrode arepreferably in their reduced form, such as for example potassiumferrocyanide. The reference electrodes opening could also be loaded withan Ag/AgCl layer (e.g., by applying Ag/AgCl ink or by sputter-coatingsilver or an Ag/AgCl layer) or other reference electrode materials.

In still another aspect of the sixth embodiment, at least fiveconductive paths are delineated on the base insulating layer: one forthe BUN working electrode; two for the creatinine/creatine workingelectrodes; one for the BUN reference electrode and one for thecreatinine/creatine reference electrode. The first middle layer, orreagent holding layer, contains at least five openings for three workingelectrodes and two reference electrodes.

In still another aspect of the sixth embodiment, at least fourconductive paths are delineated on the base insulating layer: one forthe BUN working electrode; two for the creatinine/creatine workingelectrodes; one for the combined BUN and creatinine/creatine referenceelectrode, although there are two reference electrode openings which maybe loaded with different chemical reagents. The first middle layer, orreagent holding layer, contains at least five openings for three workingelectrodes and two reference electrodes. The BUN reference electrodeopening is preferably loaded with an oxidized form of redox mediator,such as potassium ferricyanide and other ingredients; thecreatinine/creatine reference electrode opening is preferably loadedwith a reduced form of redox mediator, such as potassium ferrocyanide,and other ingredients. Note that there are two reference electrodeopenings and they may be loaded with different chemical reagents.

In the seventh embodiment of the present invention, the sensor of thepresent invention has a similar structure to the fifth embodiment, butat least six conductive paths are delineated on the base insulatinglayer: one for the BUN working electrode (W1); one for the BUN blankelectrode (B); one for the creatinine/creatine working electrodes (W2);one for creatine working electrode (W3); one for the combined BUNreference electrode (R1) and one for the creatinine/creatine referenceelectrode (R2). Accordingly, the first middle layer, or reagent holdinglayer, contains at least six openings for three working electrodes, oneblank electrode and two reference electrodes.

In one aspect of the seventh embodiment, one opening contains electrodematerial for the first working electrode (W1) loaded with urease andother ingredients; one for the blank electrode (B); one for the secondworking electrode (W2) loaded with creatinine/creatine sensitive enzymesand other ingredients; one for the third working electrode (W3) loadedwith creatine sensitive enzymes and other ingredients; one for the BUNreference electrode (R1) and one for the creatinine/creatine referenceelectrode (R2).

In the eighth embodiment of the present invention, the sensor of thepresent invention has two channels (Channel 1 and Channel 2) on the samestrip, arranged side by side. At least one channel serves as the ureasensor having a similar structure to those mentioned in the aboveembodiments; at least one channel serves as the creatinine sensor. Thesample entrance ends, or sampling entrances of the two channels, areclose to each other; or the two channels simply share the same samplingentrance. In either case, the two channels are able to use the same dropof the blood sample.

In one aspect of the eighth embodiment, the number of the baseconductive paths on the base insulating layer should match the totalnumber of the electrodes in Channel 1 and Channel 2. There are two rowsof openings on the second middle layer, one used for Channel 1 and theother one for Channel 2. Accordingly, the second middle layer has twoU-shape cutouts, one used for the Channel 1 and the other one forChannel 2. The laminated body also has a top layer with a vent openingfor each channel. The two channels can also share one larger ventopening. Preferably each has an entrance notch at the sample entranceend. More preferably, the two channels share the same entrance notch, sothat two channels are able to use the same drop of the blood sample.

In another aspect of the eighth embodiment, Channel 1 has at least oneworking electrode and one reference electrode. At least one of theworking electrodes is loaded with urease and other ingredients. Channel1 can function independently as a urea sensor.

In still another aspect of the eighth embodiment, Channel 2 has at leasttwo working electrodes and one reference electrode. One of the workingelectrodes is loaded with creatinine/creatine sensitive enzymes andother ingredients and the other one is loaded with creatine sensitiveenzymes. Channel 2 can function independently as a creatinine sensor.However, if creatine interference is not a concern, Channel 2 mayincorporate only one working electrode for the creatinine/creatinesensitive enzymes.

In a ninth embodiment of the present invention, the sensor of thepresent invention has two channels (Channel 1 and Channel 2) on the samestrip, arranged back to back (or top/bottom arrangement). At least onechannel (e.g. Channel 1) serves as the urea sensor having a similarstructure to those mentioned in the above embodiments; at least onechannel (e.g. Channel 2) serves as the creatinine sensor. The sampleentrance ends, or sampling entrances of the two channels, are close toeach other; or the two channels simply share the same sampling entrance,such that the two channels are able to use the same drop of the bloodsample. Either channel can be arranged on the top (or bottom).

In one aspect of the ninth embodiment, there are total of nine laminatedlayers. The first four layers are used for the Channel 1; last fourlayers for the Channel 2; while the fifth layer, located in the middleof the nine laminated layers, is a layer of adhesive, glue ordouble-sided tape, binding the two channels together in a back-to-backconfiguration.

In another aspect of the ninth embodiment, there are total of sevenlaminated layers. The two channels share the same base insulating layer.This base layer has conductive paths on both sides; one side used forChannel 1 and the other one for Channel 2.

In still another aspect of the ninth embodiment, Channel 1 has at leastone working electrode and one reference electrode. At least one of theworking electrodes is loaded with urease and other ingredients. Channel1 can function independently as a urea sensor.

In further aspect of the ninth embodiment, Channel 2 has at least twoworking electrodes and one reference electrode. One of the workingelectrodes is loaded with creatinine/creatine sensitive enzymes andother ingredients and the other one is loaded with creatine sensitiveenzymes. Channel 2 can function independently as a creatinine sensor.

In yet another embodiment of the present invention, the disposable striphas a sensor body with an open well forming a test chamber, a workingelectrode and a reference electrode within the test chamber, andelectrical contacts for electrically connecting the working electrodeand the reference electrode to a meter device. The test chamber containsa reagent on at least the working electrode where the reagent containsat least urease. The meter device must be capable of providing a biasingpotential across the working electrode and the reference electrode anddetecting a current generated by the presence of urea in a fluid sampledisposed into the open well of the disposable strip.

The above described embodiments are based on amperometric analyses.Those skilled in the art, however, will recognize that a sensor of theinvention may also utilize coulometric, potentiometric, voltammetric,and other electrochemical techniques to determine the concentration ofan analyte in a sample.

All of the advantages of the present invention will be made clearer uponreview of the detailed description, drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the present inventionshowing the test strip.

FIG. 2 is an exploded view of the embodiment in FIG. 1 showing the fourcomponent layers of the test strip.

FIG. 3 is a perspective view of another embodiment of the presentinvention showing the combination of a BUN sensor strip and a creatininesensor strip.

FIG. 4 is an exploded view of the embodiment in FIG. 3 showing thearrangement of the component layers of the BUN sensor strip and thecreatinine sensor strip.

FIG. 5 is a perspective view of another embodiment of the presentinvention showing the combination of a BUN sensor strip and a creatininesensor strip where the base layer is common to both sensors.

FIG. 6 is an exploded view of the embodiment in FIG. 5 showing thearrangement of the component layers of the BUN sensor and the creatininesensor.

FIG. 7 is a perspective view of another embodiment of the presentinvention showing a combined sensor strip having four workingelectrodes, namely a BUN measuring electrode, a blank working electrode,a creatinine measuring electrode, a creatine working electrode, and areference electrode.

FIG. 8 is an exploded view of the embodiment in FIG. 7 showing thearrangement of the component layers.

FIG. 9 is a perspective view of another embodiment of the presentinvention showing a combined sensor strip having three workingelectrodes namely, a BUN measuring electrode, a creatinine measuringelectrode and an interferent-compensating electrode.

FIG. 10 is an exploded view of the embodiment in FIG. 9 showing thearrangement of the component layers that includes the BUN measuringelectrode, creatinine measuring electrode, the interferent-compensatingelectrode, and a reference electrode.

FIG. 11 is a perspective view of another embodiment of the presentinvention showing a combined sensor strip having a BUN sensor systemside-by-side with the creatinine sensor system.

FIG. 12 is an exploded view of the embodiment in FIG. 9 showing thearrangement of the component layers that includes the BUN sensor systemand the creatinine sensor system.

FIG. 13 is a perspective view of another embodiment of the presentinvention showing the BUN sensor of the present invention.

FIG. 14 shows the correlation of the readings obtained using the presentinvention with a Ag—AgCl reference electrode to readings obtained withan analytical chemistry system.

FIG. 15 shows the correlation of the readings obtained using the presentinvention with a redox mediator-based reference electrode to readingsobtained with an analytical chemistry system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment(s) of the present invention is illustrated inFIGS. 1-15. FIG. 1 shows a urea sensor 10 of the present invention.Sensor 10 has a laminated body 12, a fluid sampling end 14, anelectrical contact end 16, and a vent opening 52. Sensor 10 may alsoinclude an optional inlet notch 54. Fluid sampling end 14 includes asample chamber 17 between a sampling end inlet 18 and vent opening 52.Electrical contact end 16 has three discrete conductive contacts 16 a,16 b and 16 c.

Referring now to FIG. 2, laminated body 12 is composed of a base layer20, a reagent holding layer 30, a channel forming layer 40, and a cover50. All layers of laminated body 12 are made of a dielectric material,preferably plastic. Examples of a preferred dielectric material arepolyvinyl chloride, polycarbonate, polysulfone, nylon, polyurethane,cellulose nitrate, cellulose propionate, cellulose acetate, celluloseacetate butyrate, polyester, polyimide, polypropylene, polyethylene andpolystyrene.

Base layer 20 has a conductive layer 21 on which is delineated threeconductive paths 22, 24 and 26. The conductive paths 22, 24 and 26 maybe formed by scribing or scoring conductive layer 21, or bysilk-screening conductive paths 22, 24 and 26 onto base layer 20.Scribing or scoring of conductive layer 21 may be done by mechanicallyscribing the conductive layer 21 sufficiently to create at least threeindependent conductive paths 22, 24 and 26. The preferred scribing orscoring method of the present invention is done by using a carbondioxide laser, a YAG laser or an eximer laser. An additional scoringline 28 (enlarged and not to scale; for illustrative purposes only) maybe made, but is not necessary to the functionality of sensor 10, alongthe outer edge of base layer 20 in order to avoid potential staticproblems which could give rise to a noisy signal. Conductive layer 21may be made of any electrically conductive material such as, forexample, gold, tin oxide/gold, palladium, other noble metals or theiroxides, or carbon film compositions. The preferred electricallyconductive material is gold or tin oxide/gold. A usable material forbottom layer 20 is a tin oxide/gold polyester film (Cat. No. FM-1) or agold polyester film (Cat. No. FM-2) sold by Courtaulds PerformanceFilms, Canoga Park, Calif.

Reagent holding layer 30 has a first electrode opening 32 which exposesa portion of first conductive path 22, a second electrode opening 34which exposes a portion of second conductive path 24, and a thirdelectrode opening 36 which exposes a portion of third conductive path26. Reagent holding layer 30 is made of a plastic material, preferably amedical grade, one-sided adhesive tape available from Adhesive Research,Inc., of Glen Rock, Pa. Acceptable thicknesses of the tape for use inthe present invention are in the range of about 0.001 in. (0.025 mm) toabout 0.01 in. (0.25 mm). One such tape, Arcare® 7815 (about 0.003 in.(0.075 mm)), is preferred due to its ease of handling and its ability topromote capillary action through the sample chamber of the sensor. Itshould be understood that the use of a tape is not required. Reagentholding layer 30 may be made from a plastic sheet and may be coated witha pressure sensitive adhesive, a photopolymer, ultrasonically-bonded tobase layer 20, or silk-screened onto the base layer 20 to achieve thesame results as using the polyester tape mentioned.

The three electrode openings 32, 34 and 36 define electrode wells W, Rand B, respectively, and hold chemical reagents forming a workingelectrode, a reference electrode, and a blank electrode. Generally,electrode well W is loaded with a BUN reagent matrix that contains theurea sensitive enzyme, urease, but no redox mediator and preferably atleast a polymer binder, a surfactant and a buffer. Electrode well B isloaded with a similar chemistry to W, without adding the urea sensitiveenzyme, urease, or redox mediator. One or more chemical components suchas polymers, stabilizers, and bulking agents may be optionally includedin the BUN reagent matrix and/or the blank reagent matrix. A referencematrix is loaded in electrode well R.

Typically, the reference matrix contains at least a redox reagent/coupleor mediator such as, a reduced form of redox mediator, an oxidized formof redox mediator, or a mixture of a reduced and oxidized form of redoxmediators. For example, a mixture of potassium ferricyanide andpotassium ferrocyanide may be loaded to make the reference electrodefunction when using the preferred conductive coating material. Themixture of potassium ferricyanide and potassium ferrocyanide may beprepared such that the potassium ferricyanide concentration is in therange of about 5% to about 10%, preferably 10%, while the potassiumferrocyanide concentration is in the range of about 0% to about 5%. If Ris not loaded with a redox reagent/couple or mediator, working electrodeW will not function properly. In the alternative, the referenceelectrode (electrode well R) may be loaded with a Ag/AgCl layer (e.g.,by applying Ag/AgCl ink or by sputter-coating (a) a Ag layer followed bychloridizing the Ag or (b) a Ag/AgCl layer) or other reference electrodematerials that do not require a redox mediator to function properly.

The preferred shape of the reagent holding openings is round and has apreferred diameter of about 0.03 in. (0.76 mm). The three reagentholding/electrode openings 32, 34 and 36 are aligned with each other andare spaced preferably about 0.025 in. (0.64 mm) from each other. Thecircular reagent holding openings are for illustrative purposes only. Itshould be understood that the shape and size of the reagent holdingopenings as well as the distance between the reagent holding openingsare not critical, provided that the size of the openings is big enoughto facilitate dispensing chemical reagents but small enough to allow fora reasonably small sample channel.

The positional arrangement of the working electrodes and the referenceelectrode in the sample chamber is not critical for obtaining usableresults from the BUN sensor. The possible electrode arrangements withinthe sample chamber may be W-B-R, W-R-B, R-W-B, B-W-R, B-R-W, or R-B-W,with the arrangement listed as the arrangement of electrodes wouldappear from the sample inlet 18 of laminated body 12 to the vent opening52. The preferred position was found to be W-R-B; that is, as the fluidsample enters sampling end 14 of laminated body 12, the fluid samplewould cover W first, then R followed by B. Such an arrangement may bebeneficial for obtaining usable results when the sample size isinsufficient or partially insufficient.

The working electrode, the blank electrode and the reference electrodeare all in electric contact with separate conductive paths. The separateconductive paths terminate and are exposed for making an electricalconnection to a reading device on the end opposite the sample inlet 18of laminated body 12.

Channel forming layer 40 has a U-shaped cutout 42 located at the fluidsampling end 14. The length of cutout 42 is such that when channelforming layer 40 is laminated to reagent holding layer 30, electrodeareas W, R and B are within the space defined by cutout 42. The length,width and thickness of the U-shaped cutout 42 define the capillarychamber volume. The thickness of channel forming layer 40 can affect thespeed of the sample fluid flow into the sample chamber, which is filledby capillary action of the sample fluid. Channel forming layer 40 ismade of a plastic material, preferably a medical grade, double-sidedpressure-sensitive adhesive tape available from Adhesive Research, Inc.,of Glen Rock, Pa. Acceptable thicknesses of the tape for use in thepresent invention are in the range of about 0.001 in. (0.025 mm) toabout 0.010 in. (0.25 mm). One such tape is Arcare® 7840 (about 0.0035in. (0.089 mm)). U-shaped cutout 42 can be made with a laser or bydie-cutting. The preferred method is to die-cut the cutout. Thepreferred size of the U-shaped cutout is about 0.201 in. long (5.12 mm),about 0.05 in. wide (1.27 mm), and about 0.005 in. thick (0.127 mm).

Cover 50, which is laminated to channel forming layer 40, has ventopening 52 spaced from fluid sampling end 14 of BUN sensor 10 to ensurethat the sample in the sample chamber 17 will completely cover electrodeareas W, R and B. Vent opening 52 is positioned in cover 50 so that itwill align somewhat with U-shaped cutout 42. Preferably, vent opening 52will expose a portion of and partially overlay the bottom of theU-shaped cutout 42. The preferable shape of vent opening 52 is arectangle with dimensions of about 0.08 in. (2 mm) by about 0.035 in.(0.9 mm). The preferred material for cover 50 is a polyester film. Inorder to facilitate the capillary action, it is desirable for thepolyester film to have a highly hydrophilic surface that faces thecapillary chamber. Transparency films (Cat. No. PP2200 or PP2500) from3M are the preferred material used as the cover in the presentinvention. Cover 50 may optionally include inlet notch 54.

FIG. 3 shows a combination of a BUN sensor 10 and a creatinine sensor300. Both BUN sensor 10 and creatinine sensor 300 are made of the4-layer construction where the bottoms of each sensor are laminated toeach other forming an integrated BUN/creatinine sensor combination. Eachsensor has a laminated body 12, 312, a fluid sampling end 14, 314, anelectrical contact end 16, 316, and a vent opening 52, 352 (not shown).Fluid sampling ends 14, 314 include sample chambers (not shown) betweensampling end inlets 18, 318 and vent openings 52, 352, respectively.

Turning now to FIG. 4, each sensor 10, 300 has a base layer 20, 320, areagent holding layer 30, 330, a chamber forming layer 40, 340, and acover 50, 350. Reagent holding layers 30, 330 have reagent holdingopenings 32, 34, 36 and 332, 334, 336, respectively. Channel forminglayers 40, 340 have U-shaped cutouts 42, 342, respectively. Typically,an adhesive is used to hold sensors 10 and 300 together. Preferably, anadditional layer (not shown) with adhesive on both sides is used tofacilitate assembly of sensor 10 to sensor 300.

FIG. 5 illustrates a BUN and a creatinine sensor combination with a7-layer laminated body 212. The combination includes a BUN sensor 200and a creatinine sensor 200′. Laminated body 212 includes a fluidsampling end 214, an electrical contact end 216 and vent openings 252,252′ (not shown). Fluid sampling end 214 includes two sample chambers(not shown: one between sampling end inlet 218 and vent opening 252 andthe other between sampling end inlet 218′ and vent opening 252′ (notshown).

FIG. 6 shows an expanded view of laminated body 212 of the embodiment inFIG. 5. Laminated body 212 has a central, base layer 220 with aconductive coating 221, 221′ on each side, delineating the conductivepaths for the working and reference electrodes of each sensor. Each sideof central, base layer 220 includes a reagent holding layer 230, 230′, achannel forming layer 240, 240′, and a cover 250, 250′. Reagent holdinglayers 230, 230′ have reagent holding openings 232, 234, 236 and 232′,234′, 236′, respectively. Channel forming layers 240, 240′ have U-shapedcutouts 242, 242′, respectively.

FIG. 7 illustrates yet another embodiment of the present inventionshowing a combination BUN/creatinine sensor. FIG. 7 shows a combinationBUN/creatinine sensor 600 with a laminated body 612, a fluid samplingend 614, an electrical contact end 616, and a vent opening 652. Sensor600 may optionally include an inlet notch 654. Fluid sampling end 614includes a sample chamber 617 between sample inlet 618 and vent opening652.

FIG. 8 shows an expanded view of laminated body 612 of the embodiment inFIG. 7. Laminated body 612 has a base layer 620, a reagent holding layer630, a channel forming layer 640 with a U-shaped cutout 642, and a cover650 with an optional inlet notch 654. Base layer 620 has a conductivelayer 621 on which is delineated at least five conductive paths 622,624, 626, 628, and 629. Reagent holding layer 630 has at least fivereagent holding openings 632, 634, 636, 638, and 639. Reagent holdingopening 632 exposes a portion of conductive path 622; reagent holdingopening 634 exposes a portion of conductive path 624; reagent holdingopening 636 exposes a portion of conductive path 626; reagent holdingopening 638 exposes a portion of conductive path 628; and reagentholding opening 639 exposes a portion of conductive path 629; allforming respective electrode wells.

The five reagent holding openings 632, 634, 636, 638, and 639 defineelectrode areas W1, W2, R, W3, and B, respectively, and hold chemicalreagents forming a first working electrode, a second working electrode,a third working electrode, a reference electrode and a blank electrode.Generally, electrode well W1 is loaded with a BUN reagent mixture;electrode well W2 is loaded with a creatine reagent mixture; electrodewell W3 is loaded with a creatinine/creatine reagent mixture; electrodewell B is loaded with a blank reagent mixture; and electrode well R isloaded with a reference mixture. For usable compositions of the creatineand creatinine reagent mixtures, please refer to U.S. Pat. No. 6,767,441which is incorporated herein by reference. The BUN reagent matrix, theblank reagent mixture, and the reference mixture have been disclosedpreviously.

An alternative embodiment to the embodiment illustrated in FIGS. 7 and 8has only four electrodes instead of five electrodes. FIG. 9 shows acombination BUN/creatinine sensor 700 with a laminated body 712, a fluidsampling end 714, an electrical contact end 716, and a vent opening 752.Sensor 700 may also include an optional inlet notch 754. Fluid samplingend 714 includes a sample chamber 717 between sample inlet 718 and ventopening 752.

FIG. 10 shows an expanded view of laminated body 712 of the embodimentin FIG. 9. Laminated body 712 has a base layer 720, a reagent holdinglayer 730, a channel forming layer 740 with a U-shaped cutout 742, and acover 750 with an optional inlet notch 754. Base layer 720 has aconductive layer 721 on which is delineated to at least four conductivepaths 722, 724, 726, and 728. Reagent holding layer 730 has at leastfour reagent holding openings 732, 734, 736, and 738. Reagent holdingopening 732 exposes a portion of conductive path 722; reagent holdingopening 734 exposes a portion of conductive path 724; reagent holdingopening 736 exposes a portion of conductive path 726; and reagentholding opening 738 exposes a portion of conductive path 728; allforming respective electrode wells.

The four reagent holding openings 732, 734, 736, and 738 defineelectrode areas W1, W2, R, and W3, respectively, and hold chemicalreagents forming a first working electrode, a second working electrode,a third working electrode, and one reference electrode. Generally,electrode well W1 is loaded with a BUN reagent matrix; electrode well W2is loaded with a creatinine/creatine reagent mixture; electrode well Ris loaded with a reference mixture; and electrode well W3 is loaded witha creatine mixture.

Turning now to FIG. 11, there is illustrated another embodiment of thepresent invention showing a combination of a BUN sensor system and acreatinine sensor system in a side-by-side configuration. FIG. 11 showsa combination BUN/creatinine sensor 800 with a laminated body 812, afluid sampling end 814, an electrical contact end 816, and a ventopening 852. Sensor 800 may also include an optional inlet notch 854.Fluid sampling end 814 includes a first sample chamber 817 a and asecond sample chamber 817 b between sample inlet 818 and vent opening852. It should be understood that sample inlet 818 may optionally be twoinlets (one for each of the sample chambers) adjacent to each other andthat vent opening 852 may also optionally incorporate separate ventopenings for each of the fluid sample channels. In the illustratedembodiment, one of the sample chambers incorporates the BUN sensorsystem and the other sample chamber incorporates the creatinine sensorsystem.

FIG. 12 shows an expanded view of laminated body 812 of the embodimentin FIG. 11. Laminated body 812 has a base layer 820, a reagent holdinglayer 830, a chamber forming layer 840 with a fork-shaped cutout 842having a first leg 842 a and a second leg 842 b that form samplechambers 817 a, 817 b, respectively, and a cover 850 with an optionalinlet notch 854. Base layer 820 has a conductive layer 821 on which isdelineated a plurality of conductive paths 822, 824, 826, 827, 828, and829.

Reagent holding layer 830 has six reagent holding openings 832, 834,836, 837, 838, and 839. Reagent holding opening 832 exposes a portion ofconductive path 822; reagent holding opening 834 exposes a portion ofconductive path 824; reagent holding opening 836 exposes a portion ofconductive path 826; reagent holding opening 837 exposes a portion ofconductive path 827; reagent holding opening 838 exposes a portion ofconductive path 828; and reagent holding opening 839 exposes a portionof conductive path 829; all forming respective electrode reagent wells.

Turning now to FIG. 13, there is illustrated another embodiment of thepresent invention showing a basic disposable BUN sensor 900. Disposablesensor 900 has a laminated body 912, a sample receiving well 914 and anelectrical contact end 916. Laminated body 912 has a base layer 920 anda cover 950. Cover 950 has a sample opening 952 that forms, whencombined with base layer 920, sample receiving well 914. Base layer 920has at least two electrical paths 922 and 924, which have a firstportion exposed at electrical contact end 916 for connection to a meterdevice (not shown) and a second portion exposed by sample receiving well914.

The second portion of electrical paths 922 and 924 exposed by samplereceiving well 914 create at least a first working electrode W1 and atleast a reference electrode R1. A BUN reagent mixture 960 (representedby the hash marks in the FIG. 13) contains at least urease and isdisposed on the first working electrode W1. As previously disclosed, thepreferred BUN reagent mixture is reagent mixture 1. The referenceelectrode R1 may contain any reference material previously disclosed. Inthis embodiment of the present invention, sample receiving well 914serves as both the sample inlet and the sample chamber for receiving afluid sample such as blood for the determination of BUN.

It should be understood that the conduit paths in any of the embodimentsdisclosed herein may be made from any non-corroding metal. Carbondeposits such as for example carbon paste may also be used as theconduit paths, all as is well known by those of ordinary skill in theart.

Enzymes

The BUN sensor of the present invention includes at least aurea-sensitive enzyme capable of reacting with urea. This enzyme isessential to obtain the electrochemical signal for urea. A commerciallyavailable urease such as that available asCat No. URH-201 from Toyobo,Osaka, Japan may be added into the reagent mixture used for the BUNworking electrode. The concentration of urease in the reagent mixture ispreferably 0.5% (W/W) to 25%. More preferably, the concentration ofurease is about 10%.

Polymers

The polymers used as binders should be sufficiently water-soluble andshould also be capable of stabilizing and binding all other chemicals inthe reagents in the electrode areas to the conductive surface layer.Preferably, at least two polymers were added in the reagent mixture ofthe present invention. One of the preferred polymers is polyethyleneoxide (PEO). Its molecular weight ranges from thousands to millions.Preferably, the molecular weight is over 1 million. More preferably, themolecular weight is about 4 million. Such a product is available fromScientific Polymer Products, NY, USA (MW 4,000,000, Cat No. 344). Theconcentration of PEO in the reagent mixture is preferably 0.04% (W/W) to2%. More preferably, the concentration of PEO is about 0.4%. The otherpolymer is preferably methylcellulose, which is available under thebrand name of Methocel 60 HG (Cat. No. 64655, Fluka Chemicals,Milwaukee, Wis., USA). The concentration of Methocel 60 HG in thereagent mixture is preferably 0.02% (W/W) to 5%. More preferably, theconcentration of Methocel 60 HG is about 0.75%.

Surfactants

The surfactant is needed only to facilitate dispensing of the reagentmixture into the openings for the working and reference electrodes, aswell as for quickly dissolving the dry chemical reagents when a sampleis applied to the channel. The amount and type of surfactant is selectedto assure the previously mentioned function and to avoid a denaturingeffect on the enzymes. Surfactants can be selected from, but are notlimited to, various anionic, cationic, non-ionic and zwitterionicdetergents, such as polyoxyethylene ether, Tween 20, sodium cholatehydrate, hexadecylpyridinium cholide monohydrate and CHAPs. Thepreferred surfactant is a polyoxyethylene ether. More preferably, it ist-octylphenoxypolyethoxyethanol and is available under the brand nameTriton X-100. The concentration of Triton X-100 in the reagent mixtureis preferably 0.01% (W/W) to 2%. More preferably, the concentration ofTriton X-100 is about 0.08%.

The Buffer

Optionally, a buffer may be present along with a redox mediator in driedform in the sensor strip of the present invention. The buffer is presentin a sufficient amount so as to substantially maintain the pH of thereagent mixtures. Examples of suitable buffers include citric acid,phosphates, Tris and the like. In the present invention, a concentrationrange of about 5 mM to 150 mM Tris buffer with a pH of about 8 isemployed to prepare the reagent mixtures.

Bulking Reagent

A water soluble and inactive ingredient or bulking agent is preferablyadded into the reagent mixture, such that the electrode openings willnot trap bubbles when a sample fluid fills the capillary channel.Various sugars, such as trehalose, galactose, glucose, sucrose, lactose,mannitol, mannose, fructose, sucrose, lactose, lactitol, sorbitol,xylitol maltose, etc., can be added into the reagent mixture as long asthey do not react with other ingredients and are inactive at theelectrode surface. The preferred sugar is sorbitol. The concentration ofsorbitol in the reagent mixture is preferably 0.5% (W/W) to 25%. Morepreferably, the concentration of D(+) trehalose is about 10%.

Redox Mediators

The redox mediators are not necessary to obtain the BUN signal for theBUN working electrode. The redox mediators can be included at theworking electrode and, if included, an oxidized form of the redoxmediators such as potassium ferricyanide is preferred. The redoxmediators are necessary for the reference electrodes when the preferredreference reagent mixtures are used in order for the reference electrodeto function properly. It is desirable that the mediator is stable in thematrix. It is still desirable that the mediator can maintain a desiredpotential for the BUN working electrode. The mediator can be selectedfrom, but not limited to, various metal complexes and organic redoxcompounds, such as potassium ferricyanide, ferrocene and itsderivatives, promazine, tetrathiafulvalene, methyl blue,1,4-benzoquinone, 1,4-bis(N,N-dimethylamino) benzene, and4,4′-dihydrobiphenyl. The preferred mediator in the present invention isan oxidized form, such as potassium ferricyanide (K₃Fe(CN)₆). Theconcentration of potassium ferricyanide in the reagent mixture ispreferably 0.5% (W/W) to 15%. More preferably, the concentration ofpotassium ferricyanide is about 10%. It should be noted that a silver orAg/AgCl layer or other reference electrode materials can be applied tothe reference electrode opening, which do not require the use of anadditional redox mediator such as those listed above.

Accordingly, the reagent mixture (referred to below as “reagent mixture1”) used for the BUN working electrode (W) contains 0.75% (W/W) Methocel60 HG, 0.4% (W/W) polyethylene oxide, 0.08% (W/W) Triton X-100, 10%sorbitol, 5% (W/W) urease and 20 mM Tris buffer (pH 8). The reagentmixture (referred to below as “reagent mixture 2”) used for the blankelectrode (B) contains 0.75% (W/W) Methocel 60 HG, 0.4% (W/W)polyethylene oxide, 0.08% (W/W) Triton X-100, 10% sorbitol, and 20 mMTris buffer (pH 8). The reagent mixture (referred to below as “reagentmixture 3”) used for the BUN reference electrode contains 0.75% (W/W)Methocel 60 HG, 0.4% (W/W) polyethylene oxide, and 0.08% (W/W) TritonX-100, 10% (W/W) potassium ferricyanide, and 20 mM Tris buffer (pH 8).

Preparation of the Reagent Mixtures

Reagent mixture 1 was preferably prepared in two steps, although it canbe prepared in one step:

Step 1: Into 100 ml of 20 mM Tris buffer (pH 8), add 0.75 g Methocel 60HG, 0.4 g polyethylene oxide, 10.0 gram sorbitol, and 0.08 g TritonX-100. Stir the solution until dissolved.

Step 2: Into the above solution, add 5 g urease. Stir the solution untildissolved. The resulting solution is ready for dispensing.

Reagent mixture 2 was prepared as below:

Into 100 ml of 20 mM Tris buffer (pH 8), add 0.75 g Methocel 60 HG, 0.4g polyethylene oxide, 10.0 gram sorbitol, and 0.08 g Triton X-100. Stirthe solution until dissolved.

Reagent mixture 3 was prepared in two steps:

Step 1: Into 100 ml of 20 mM Tris buffer (pH 8), add 0.75 g Methocel 60HG, 0.4 g polyethylene oxide, and 0.08 g Triton X-100. Stir the solutionuntil dissolved.

Step 2: Into the above solution, add 10 g potassium ferricyanide. Stirthe solution until dissolved. The resulting solution is ready fordispensing.

Sensor/Strip Construction

Assembly of the various embodiments of the present invention isrelatively straightforward. Generally for the 4-layer configuration, thebase layer and reagent holding layer are laminated to each otherfollowed by dispensing the reagent mixtures into their respectiveelectrode wells. After drying the reagent mixtures, the channel forminglayer is laminated onto the reagent holding layer and the cover is thenlaminated onto the channel forming layer. For the combination sensorconstruction, the base layer and the reagent holding layer are laminatedto each other followed by dispensing the reagent mixtures within theirrespective electrode wells (or within each of the electrode wells in thelegs of the side-by-side embodiment). After drying the reagent mixture,the channel forming layer is laminated onto the reagent holding layerand the cover is then laminated onto the channel forming layer.

More particularly, a piece of a gold polyester film is cut to shape asillustrated in FIG. 2, forming base layer 20 of sensor 10. A laser(previously disclosed) is used to score the gold polyester film. Asillustrated in FIG. 2, the film is scored by the laser such that threeelectrodes at sample fluid end 14 and three contact points 22, 24 and 26are formed at electrical contact end 16. The scoring line is very thinbut sufficient to create three separate electrical paths. A scoring line28 may optionally be made, but is not necessary, along the outer edge ofbase layer 20 to avoid potential static problems which could cause anoisy signal from the finished sensor 10.

A piece of one-sided adhesive tape is then cut to size and shape,forming reagent holding layer 30 so that it will cover a major portionof conductive layer 21 of base layer 20 except for exposing a smallelectrical contact area illustrated in FIG. 1.

Before attaching reagent holding layer 30 to base layer 20, threecircular openings 32, 34 and 36 of substantially equal size are punchedby laser, or by mechanical means such as a die-punch assembly, creatingelectrode openings 32, 34 and 36 in reagent holding layer 30. Thepreferred hole size for opening 32, 34 and 36 has a typical diameter ofabout 0.030 in. (0.76 mm). As illustrated in FIG. 2, electrode openings32, 34 and 36 are aligned with each other and have a spacing of about0.025 in (0.63 mm) between them. The circular openings are forillustrative purposes only. It should be understood that the shape andsize of the openings and the distance between the openings are notcritical, provided that the size of the openings is big enough to holdsufficient chemical reagents for the electrodes to function properly butsmall enough to allow for a reasonably small sample chamber. As statedpreviously, the preferred arrangement of the electrodes formed inopenings 32, 34 and 36 is W (working electrode), R (reference electrode)and B (blank electrode). Reagent holding layer 30 is then attached tobase layer 20 in such a way as to define the electrode wells W, R and B.A quantity of silver/silver chloride ink is added into electrode well R(opening 34) and then the partially assembled sensor strip is preferablyheated/baked at about 50° C. for about 1 hour, forming the referenceelectrode.

Following creation of the reference electrode, approximately 0.05 to0.09 μL of reagent mixture 1 is dispensed into electrode area W. Asdescribed above, reagent mixture 1 is preferably a mixture of an enzyme,a stabilizer, a binder, a surfactant, and a buffer. Similarly, reagentmixture 2 is dispersed into electrode area B. Reagent mixture 2 ispreferably similar to reagent mixture 1 but without the presence of theenzyme urease.

After the addition of the reagents, the reagents are dried. Drying ofthe reagents can occur within a temperature range of about roomtemperature to about 60° C. The length of time required to dry thereagents is dependent on the temperature at which the drying process isperformed. Drying at a higher temperature for too long a period of timereduces the activity of the enzyme.

It should be understood that if a reference reagent containing a redoxmediator/couple is used in place of the silver/silver-chloride ink, thereference reagent may be disposed in the reference well at the same timeReagents 1 and 2 are disposed in the working electrode wells and allthree reagents dried simultaneously at the same drying conditions.

After drying, a piece of double-sided tape available from AdhesiveResearch is fashioned into chamber forming layer 40 containing U-shapedchannel 42. Chamber forming layer 40 is then layered onto reagentholding layer 30. As mentioned earlier, this chamber forming layer 40serves as a spacer and defines the size of the sample chamber 17. Itswidth and length are optimized to provide for a relatively quick movingfluid sample.

A piece of a transparency film (Cat. No. PP2200 or PP2500 available from3M) is fashioned into top layer 50. A rectangular vent opening 52 ismade using the laser previously mentioned or by means of a die-punch.Vent opening 52 is located approximately 0.180 in. (4.57 mm) from fluidentrance 54. Top layer 50 is aligned and layered onto chamber forminglayer 40 to complete the assembly of sensor 10, as illustrated in FIG.1.

Although the description of electrode construction above describesconstruction for a single sensor, the design and materials used areideal for making multiple sensors from one piece of each layer material.This would be accomplished by starting with a relatively large piece ofbase layer having a conducting layer thereon. A plurality of scoredlines are made into the conductive layer such that a repetitive patternis created using the preferred scribing method previously described,whereby each pattern will eventually define the conductive paths foreach sensor. Similarly, a large piece of the reagent holding layermaterial also having a plurality of openings in a repetitive pattern issized to fit over the base layer in such a way that a plurality ofsensors will be made when completed. The size of each aperture and theelectrode material disposed in the plurality of electrode areas W, R andB are similar to that disclosed above. After disposing the reagentmixture in their respective reagent holding openings and drying process,a large piece of the channel forming layer material having a pluralityof elongated apertures is layered onto the reagent holding layermaterial such that each elongated aperture of the channel forming layermaterial contains corresponding openings of the reagent holding layermaterial. A comparably-sized cover layer material having a plurality ofvent openings and notch-forming openings in a repetitive pattern islayered onto the chamber forming layer material. The laminated sheet isthen cut in appropriate locations to form individual urea sensors.

The following examples illustrate the unique features of the presentinvention. All sensors of the present invention were tested on abreadboard urea meter manufactured by Nova Biomedical Corporation ofWaltham, Mass. A potential of approximately 0.80 Volts was appliedacross the working electrodes and the reference electrode when thereference electrode was a silver/silver-chloride electrode and theresultant current signals were converted to urea concentrations. Thereadings were compared to readings (control readings) obtained on thesame samples using a Vitros® Chemistry System (Model DT 6011,Ortho-Clinical Diagnostic, Rochester, N.Y.).

A potential of approximately 0.4 Volts was applied across the workingelectrodes and the reference electrodes when the reference electrode wasa redox mediator/couple in a reagent matrix and the resultant currentsignals were converted to urea concentrations. The readings werecompared to readings (control readings) obtained on the same samplesusing a Dimension® clinical chemistry system (Model RxL, Dade Behring,Inc.).

EXAMPLE 1 Demonstration of Determination of Urea Concentration withAg—AgCl Reference

Blood samples with different urea concentrations were tested with theurea sensors of the present invention having a silver-silver chloridereference electrode and with the Vitros® Chemistry System DT6011(Ortho-Clinical Diagnostics, Inc., Rochester, N.Y.). FIG. 14 shows thedetermination of BUN concentration in mg/dL in blood samples using theurea sensors of the present invention to varying urea concentrations inthe blood samples. It is noted that these determinations are raw databefore any compensation for hematocrit, in order to show the actualcorrelation of the unmanipulated data.

As seen from the graph, the sensors of the present invention respond tothe urea concentration in the blood samples over a tested range of about17 mg/dL to about 139 mg/dL of blood urea nitrogen. The averagecoefficient of variation for the sensors of the present invention usinga silver/silver-chloride reference electrode is about 4.5%.

EXAMPLE 2 Demonstration of Determination of Urea Concentration withRedox Reference

Blood samples with different urea concentrations were tested with theurea sensors of the present invention having a redox mediator/couplereference matrix and with a Dimension® Chemistry System (Dimension RxL,Dade Behring, Inc.). FIG. 15 shows the determination of BUNconcentration in mg/dL in blood samples using the urea sensors of thepresent invention to varying urea concentrations in the blood samples.

As seen from the graph, the sensors of the present invention respond tothe urea concentration in the blood samples over a tested range of about19 mg/dL to about 187 mg/dL of blood urea nitrogen. The averagecoefficient of variation for the sensors of the present invention havinga redox mediator/couple based reference is about 6.4%.

Note that the testing results can be corrected for hematocritinterference. The hematocrit of the blood sample can be calculated basedon the impedance measurements between two electrodes, which has beendisclosed in the following US published patents and patent applications(U.S. Pat. Nos. 6,767,441; 6,287,451; 6,837,976).

Although the preferred embodiments of the present invention have beendescribed herein, the above description is merely illustrative. Furthermodification of the invention herein disclosed will occur to thoseskilled in the respective arts and all such modifications are deemed tobe within the scope of the invention as defined by the appended claims.

What is claimed is:
 1. A system for determining the urea nitrogen tocreatinine ratio in a sample, said system comprising: a disposable ureasensor comprising a laminated body having a fluid sample inlet end andan electrical contact end, said laminated body includes a base layer, areagent holding layer with cutouts defining a working electrode and areference electrode, a channel forming layer, and a cover, and adissolvable mixture of a urea reagent matrix disposed in a cutout anddirectly on said working electrode wherein said urea reagent matrixcontains urease but absent any redox mediator; a disposable creatininesensor attached to said disposable urea sensor; means for determining aurea concentration in a portion of said sample using said disposableurea sensor; means for determining a creatinine concentration in aportion of said sample using said disposable creatinine sensor; andmeans for using said urea concentration and said creatinineconcentration to determine said urea nitrogen to creatinine ratio ofsaid sample.
 2. The system of claim 1 wherein said urea reagent matrixfurther includes one or more of a material selected from the groupconsisting of a binder, a buffer, a surfactant, a stabilizer, and anbulking reagent.
 3. A combination BUN-creatinine sensor comprising: afirst working electrode for measuring urea having a dissolvable mixtureof a BUN reagent matrix disposed directly thereon, wherein said BUNreagent matrix includes urease but absent any redox mediator; a secondworking electrode for measuring creatinine with a creatinine reagentmatrix disposed thereon; a third working electrode for measuringcreatine having a creatine reagent matrix disposed thereon; a referenceelectrode wherein said first working electrode, said second workingelectrode, said third working electrode, and said reference electrodeare formed by a reagent holding layer having a separate cutout for eachof said first working electrode containing said BUN reagent matrix, saidsecond working electrode containing said creatinine reagent matrix, saidthird working electrode containing said creatine reagent matrix, andsaid reference electrode containing a redox mediator; and a samplechamber containing said first working electrode, said second workingelectrode, said third working electrode and said reference electrode. 4.The combination sensor of claim 3 wherein said BUN reagent matrixfurther includes one or more of a material selected from the groupconsisting of a binder, a buffer, a surfactant, a stabilizer, and abulking reagent.
 5. A method for determining the blood urea nitrogen tocreatinine ratio of a blood sample, said method comprising: obtaining acombination disposable sensor containing a blood urea nitrogen electrodecontaining a BUN reagent matrix having urease but absent any redoxmediator, creatinine and creatine electrodes, and a reference electrodecontaining a redox mediator but no urease formed by a reagent holdinglayer having a separate cutout for each of said blood urea nitrogenelectrode, said creatinine and creatine electrodes and said referenceelectrode, said combination having at least one sample inlet, at leastone sample chamber and electrical connection means; disposing a portionof said blood sample into said at least one sample chamber; measuringthe blood urea nitrogen concentration of said blood sample; measuringthe creatinine concentration of said blood sample; and determining theblood urea nitrogen to creatinine ratio.
 6. The method of claim 5wherein said obtaining step further includes making a disposable bloodurea nitrogen sensor, making a disposable creatinine sensor andattaching said disposable blood urea nitrogen sensor to said disposablecreatinine sensor in a back-to-back configuration.
 7. The method ofclaim 5 wherein said obtaining step further includes obtaining a baselayer having a plurality of electrically conductive paths on oppositesides of said base layer and assembling a blood urea nitrogen sensor anda reference electrode on one side of said base layer and assembling acreatinine sensor and a reference electrode on an opposite side of saidbase layer.
 8. The method of claim 5 wherein said obtaining step furtherincludes obtaining a base layer having a plurality of electricallyconductive paths on one side of said base layer, assembling said bloodurea nitrogen sensor and said reference electrode using at least twoconductive paths of said plurality of electrically conductive paths andassembling a creatinine sensor using at least two other conductive pathsof said plurality of electrically conductive paths wherein said bloodurea nitrogen sensor and said creatinine sensor are disposed within saidat least one sample chamber.
 9. The method of claim 5 wherein saidobtaining step further includes obtaining a base layer having aplurality of electrically conductive paths on one side of said baselayer, creating a blood urea nitrogen electrode, creating a creatinineelectrode, creating a creatine electrode, and creating a referenceelectrode wherein said blood urea nitrogen electrode, said creatinineelectrode, said creatine electrode and said reference electrode aredisposed within said at least one sample chamber.